1. Field of the Invention
This invention relates to an easy-to-clean glass ceramic cooktop, preferably for use as a cooking surface for a cooktop such as on kitchen stoves, and in particular for self-cleaning ovens or warming stoves. This invention further relates to an easy-to-clean glass ceramic window, preferably for use as a window for oven doors, such as on kitchen stoves, and in particular for windows on self-cleaning ovens or warming stoves.
2. Background of the Invention
Glass ceramic plates for glass ceramic cooking surfaces such as cooktops are typically difficult to clean because the surface contains uneven spots and pits or pores in which spilled or encrusted food gathers during cooking. The food particles are difficult to remove. Even when the cooking surface is cleaned with a scraper, residue is left behind in the pits. The uneven spots on the cooking surface also minimize energy transmission from the heating source of the cooking surface to a cooking utensil such as a pot or pan because air cushions are formed between the cooking surface and the bottom of a cooking utensil. Thus, cooking time is increased, thereby decreasing the efficiency of the cooktop.
Another disadvantage of typically manufactured cooking surfaces of cooktops is that the underside of the cooking surface contains dimples that create distortion. In addition, dimples make the installation of displays and sensors difficult.
The cooking surface can be made smooth by grinding and polishing or by the application of a coating, but these additional steps increase the cost of manufacturing of the cooking surface.
Glass ceramic plates for glass ceramic cooking surfaces such as cooktops are typically manufactured by rolling Lixe2x80x94Alxe2x80x94Si glass (xe2x80x9cgreen glassxe2x80x9d), which is then ceramized. The glass ceramic material has a red to red-brown color or is transparent with a non-transparent coating on the underside. As a result of the rolling, the glass ceramic plate acquires a surface that has uneven spots from which it is difficult to remove dirt particles that typically originate from spilled or overflowing food being cooked. When decoration is applied to the cooking surfaces, individual areas of the decor also stick up and are the first parts to be rubbed off when mechanical stress is applied. The surface of the cooking surface is also not flat.
The Ra of the roughness (arithmetic mean of the absolute height differences from the center plane) is 0.35 xcexcm to 0.55 xcexcm. In spite of this roughness, a scraper blade is the most effective cleaning agent. It cleans better than sponges and cleaning agents with abrasive particles. However, it leaves residues behind in the xe2x80x9cdepressionsxe2x80x9d.
The glass ceramic plates have a deviation from flatness of up to xc2x10.4 mm and a size of 300xc3x97300 mm. The resulting distortions of the images of long sources of light such as fluorescent tubes are noticeable, however, and adversely affect the high-quality image of the product.
The uneven spots are also disadvantageous with regard to energy transmission. The energy transmission to the pots and pans on cooking units with glass ceramic cooking surfaces is primarily by thermal transmission. Only the transport of heat from the spiral-wound heating coil to the underside of the cooking surface occurs by radiation. As a result of uneven spots, air cushions are formed between the cooking surface and the bottom of the pot that increase the time required for cooking. They also lead to an increase in the surface temperature, as a result of which losses to the environment increase and the efficiency decreases.
Typically, the underside of the cooking surface is provided by means of a patterned roller with a dimpling that has a wavelength of approximately 2 mm and a dimple height of 50 xcexcm to 200 xcexcm.
The dimples on the underside also make it impossible to successfully use heating elements that require thermal contact.
As a result of the increased use of electronic controls in cooking devices, an increased number of displays are also being used. To preserve the smooth, flat impression of the unit, these displays are installed under the glass ceramic. On account of the dimpling of the material, these displays are visible only with distortions.
The installation of sensors on the underside is made more difficult by the dimpling, because when the screen printing process is used, it is impossible to achieve a uniform coating thickness. When the sensors on the dimpled side are pressed, the thermal contact is also poor.
A smooth surface of a cooktop made of glass ceramic can be achieved in the manner of the prior art, for example, by grinding followed by polishing. Typically, the preliminary grinding is done with a grain size of approximately 100 xcexcm. The preliminary grinding is followed by grinding with a grain size of 12-15 xcexcm. The final polishing is done with an even finer grain size.
A smooth surface can also be produced in the manner of the prior art by means of a coating. For example, there are transparent glass ceramics that have a heat-reflecting coating, e.g. SnO2. The prior art also discloses the use of SiO2 as a protective coating on a glass ceramic cooktop. Both coatings make the surface smooth and facilitate cleaning, among other things.
Although the methods of the prior art described above do produce a cooktop made of glass ceramic that has a smooth surface, they require additional steps after the manufacture of the glass ceramic cooktop that increase the manufacturing costs for the cooktop.
Windows for enclosed heated areas where the temperature exceeds 350xc2x0 C. are typically made of transparent glass ceramic. When such windows are used as oven door windows for heating stoves or in pyrolysis ovens, combustion residues are deposited on the windows and have to be removed. Commercial cleaning agents, cloths, sponges and scrapers are used to clean the windows.
It is difficult to clean a window of this type. Residue often remains on the window and cannot be removed. This difficulty of removal is caused by the composition of the residues and/or by the surface characteristics of the glass ceramic.
The surface of the transparent glass ceramic that is manufactured using conventional methods is typically structured and uneven. It is similar to an orange peel and generally has a surface with a Ra (arithmetic mean of the absolute height differences from the center plane) of 0.35 xcexcm to 0.55 xcexcm. This macroscopic structure also contains small isolated holes or depressions, pores, or pits, or elevations that have a diameter of up to 0.5 mm. The dirt accumulates in the depressions as well as behind the elevations of the surface and can no longer be removed mechanically. Mechanical cleaning using special cleaning scrapers and sponges is of only limited effectiveness for this purpose. Nevertheless, the best cleaning results are frequently achieved with a sponge and scraper, because, given the composition of the combustion residue, commercial chemical cleaning agents are no longer effective.
These cleaning problems do not occur on a window made of glass ceramic that has a smooth surface on at least one side.
A smooth surface of a window made of glass ceramic can be achieved in the manner of the prior art, for example, by grinding followed by polishing. Typically, the preliminary grinding is done with a grain size of approximately 100 xcexcm. The preliminary grinding is followed by grinding with a grain size of 12-15 xcexcm. The final polishing is done with an even finer grain size.
A smooth surface can also be produced in the manner of the prior art by means of a coating. For example, there are transparent glass ceramics that have a heat-reflecting coating, e.g. SnO2. The prior art also discloses the use of SiO2 as a protective coating on a glass ceramic window. Both coatings make the surface smooth and facilitate cleaning, among other things.
Although the methods of the prior art described above do produce a window made of glass ceramic that has a smooth surface, they require additional steps after the manufacture of the glass ceramic window that increase the manufacturing costs for the window.
As mentioned, the situation is similar for the manufacture of a glass ceramic plate that is used as a cooking surface of a cooktop.
This invention relates to an easy-to-clean glass ceramic object for household use, preferably for use as a cooking surface for a cooktop or as a window for oven doors, such as on kitchen stoves, and in particular for windows on pyrolysis ovens or heating stoves.
The invention also relates to window panes for enclosed hot areas with temperatures  greater than 350xc2x0 C., e.g. in self-cleaning ovens, which are typically made of transparent glass ceramic. When used as windows in heating stoves or in pyrolysis ovens, combustion residues are thereby deposited on the windows, which residues must then be removed. It is difficult to clean a window of this type, because the surface of the transparent glass ceramic that is manufactured using conventional methods is typically structured and uneven.
The invention further relates to a cooking surface for a cooktop. Encrusted, spilled, or overflowing food is deposited on the cooking surfaces of cooktops during cooking and the removal of such residue is difficult because the surface of a cooktop is typically rough and uneven. The invention teaches the manufacture of an easy-to-clean glass ceramic cooktop from a floated glass ceramic.
The invention teaches the manufacture of an easy-to-clean class ceramic window body from a floated glass ceramic that has a specified surface quality that is comparable to that of a floated soda lime glass pane.
The object of the invention is to create a glass ceramic surface for a cooktop that is easy to clean and that has a smooth and, therefore, easy-to-clean surface even without additional manufacturing steps such as polishing or coating.
Another object of the invention is to create a glass ceramic window for an oven or stove that is easy to clean and that has a smooth and, therefore, easy-to-clean surface even without additional manufacturing steps such as polishing or coating.
Another object of the invention is to create a glass ceramic object that is easy to clean and that has a smooth and, therefore, easy-to-clean surface even without additional manufacturing steps such as polishing or coating.
The invention teaches that this object can be accomplished by an easy-to-clean glass ceramic object, comprising a floated glass ceramic that directly and without additional polishing has a surface structure that has a roughness determined by the average roughness Raxe2x89xa60.02 xcexcm and/or the square average roughness Rqxe2x89xa60.01 xcexcm.
As a result of the measures claimed by the invention, it is possible to obtain a glass ceramic object with a very smooth surface structure that is identical to the surface of soda lime float glass. There is essentially no need for additional fine grinding or for coating. The structure of the surface is easily controlled from the very beginning of the process by an appropriate selection of the composition and the manufacturing method of the glass ceramic.
The green glass, i.e. the primary glass used for the object to be ceramized, is manufactured using the float process. As a result, the green glass has a smooth, flat surface. This surface remains smooth and flat during the subsequent ceramization, and on glass ceramic plates for cooking surfaces it also remains smooth and flat during the decoration process. The fine corrugations that are unavoidably formed when the glass is manufactured by rolling do not form in floated material and, as a result, the upper side of the glass is particularly flat and smooth.
As a result of the smooth surface, floated ceramic objects manufactured as claimed by the invention are easy to clean. When these glass ceramic objects are used as windows in oven doors, it is easy to remove condensed vapors as well as pyrolysis residues and similar substances. When these glass ceramic objects are used as glass ceramic cooking surfaces, it is easy to remove encrusted, spilled, or overflowed food. Cleaning using a blade scraper therefore leaves essentially no residue.
The cooking surfaces manufactured from the glass ceramic claimed by the invention have a deviation from flatness of xc2x10.03 mm, which is typical for float glass, with reference to a format of 300xc3x97300 mm (compared to 0.4 mm for rolled material). On account of their better flatness, such surfaces are characterized by a particularly advantageous optical impression with regard to the reflection of light sources in the kitchen or from the outdoors. Moreover, the decorative coating, which is conventionally approximately 5 xcexcm thick on glass ceramic cooking surfaces, is oriented in a plane, on account of the absence of fine corrugations. Because there are no elevated areas, the uneven wear of the decoration under mechanical load is prevented. Consequently, the decoration does not become xe2x80x9ccloudyxe2x80x9d after extended use.
The transmission of energy is also improved.
The terms used to describe surface roughness are defined in, among other places, German standard DIN 4762. For example, the average roughness Ra is the arithmetic mean or average of the absolute height differences from the center plane or the arithmetic average of the absolute amounts of the differences between the actual or measured profile and the average profile. This average profile is calculated by laying a profile through the measured profile within a reference length, so that the sum of the surface area of the measured profile filled with material on the top and the sum of the surface areas free of materials on the bottom are equal. On the basis of DIN 4762, Rq=square average roughness, determined by means of white light interference microscopy (measurement area 0.6xc3x970.5 mm). In terms of formulas, this concept is expressed as follows:             R      a        =                  (                              "LeftBracketingBar"                          Z              1                        "RightBracketingBar"                    +                      "LeftBracketingBar"                          Z              2                        "RightBracketingBar"                    +                      "LeftBracketingBar"                          Z              3                        "RightBracketingBar"                    +          …          +                      xe2x80x83                    ⁢                      "LeftBracketingBar"                          Z              n                        "RightBracketingBar"                          )            N                  R      q        =                            (                                    Z              1              2                        +                          Z              2              2                        +                          Z              3              2                        +            …            ⁢                          xe2x80x83                        +                          Z              n              2                                )                N            
The manufacture of flat glass ceramic objects is described by the prior art.
To simplify the manufacture of such glass ceramics using float glass as the primary material, attempts have been made to perform the ceramization as early as in the float bath as possible, to thereby obtain the glass ceramics directly. In such a process, defects in the float glass, in particular undesirable surface crystals that occur during the floating, cannot be detected and eliminated, which disadvantage has an adverse effect on the surface quality of the float glass.
Theoretically, all glass ceramics floated according to the methods of the prior art can be used for the manufacture of the easy-to-clean glass ceramic object claimed by the invention.
To achieve a particularly good surface quality and thus a correspondingly high ease of cleaning, the initial glass used for the glass ceramic is a float glass, in which the origin of undesirable surface defects during the floating is prevented by restricting the concentrations of Pt to  less than 300 ppb, Rh to  less than 3.0 ppb, ZnO to  less than 1.5 wt. and SnO2 to  less than 1 wt. %, and by fining or refining the glass during the melting without using the conventional fining agents arsenic oxide or antimony oxide.
These types of glass are therefore characterized by a composition that makes it possible to prevent the formation of undesirable surface defects during floating. Floats conventionally consist of the melting chamber or hot end, in which the glass is melted and fined or refined, an interface that provides the transition from the oxide atmosphere in the melting chamber into the reducing atmosphere in the rest of the system, and the float portion, in which the glass is shaped by pouring it onto a molten metal, generally Sn, in a reducing atmosphere of forming gas. The glass is formed by allowing it to flow out smoothly onto the Sn bath and by top rollers that exert a force on the surface of the glass. During the transport on the metal bath, the glass cools, and at the end of the float portion it is lifted off and transferred into a cooling furnace or lehr or annealing furnace/oven.
During the formation of the glass surface and the transport in the float, interactions between the glass and the float atmosphere or the Sn batch can result in undesirable surface defects.
If the glass contains more than 300 ppb Pt or more than 30 ppb Rh in dissolved form, metallic precipitations of Pt or Rh particles can form as a result of the reducing conditions in the glass surface, and these particles can serve as effective seeds for large high quartz or beta quartz mixed crystals up to 100 xcexcm large, and thus cause undesirable surface crystallization.
These materials are used in, among other things, electrodes, linings, agitators, transport tubes, valve gates etc. In plants for the performance of the method for the manufacture of the glass ceramic described above, to prevent the formation of surface crystals, therefore, components that contain Pt or Rh are completely avoided and are replaced by ceramic materials, or the conditions in the melting chamber or interface are realized so that the above-mentioned concentrations are not exceeded.
The ZnO concentration is restricted to 1.5 wt. %. It has been shown that under the reducing conditions of the floating, the zinc is depleted in the surface of the glass. It is thereby assumed that the zinc is partly reduced on the surface of the glass, whereupon it vaporizes as a result of the higher vapor pressure of Zn compared to Zn2+ in the float atmosphere. In addition to the evaporation and deposition of the Zn in colder spots, which are undesirable for the operation of the float, the uneven distribution of the Zn in the glass also participates in the origin of critical crystal bands close to the surface. These crystal bands of large high or beta quartz mixed crystals originate in the vicinity of the surface where the Zn concentration in the glass has risen back close to the initial value. It is therefore appropriate to keep the initial value low from the start.
The concentration of SnO2 in the glass is restricted to less than 1 wt. %. As a result of the action of the reducing conditions in the float portion, the SnO2 is partly reduced, especially in the surface of the glass. Surprisingly, small metal Sn spheres form in the glass in the immediate surface of the glass, and although they can easily be removed during cooling or cleaning, they leave behind spherical holes or pits or depressions that are extremely undesirable for the intended use of the glass.
These small spheres can be prevented if the concentration of SnO2 is very low.
The above-mentioned primary glasses are fined or refined without using the fining agents arsenic oxide and/or antimony oxide which are conventional for glass from the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 system. Under the action of the reducing conditions during floating, the above mentioned fining agents in particular are reduced directly on the surface of the glass and form undesirable and visible metallic coatings. The removal of these coatings, which are aesthetically and toxicologically undesirable, requires grinding and polishing and is disadvantageous for economic reasons. To prevent the formation of the coatings, it is therefore appropriate to achieve a low seed number or number of seeds or number of bubbles by adding at least one alternative chemical fining agent, such as SnO2, CeO2, sulfate compounds, chloride compounds, for example, preferably 0.2-0.6 wt. % SnO2, to the molten glass. Alternatively, the molten glass can also be fined physically, e.g. by means of underpressure or by means of high temperature  greater than 1750xc2x0 C. Thus the required seed quality or number of bubbles can be achieved by means of alternative fining agents and/or alternative fining methods.
During the ceramization, care must be taken to avoid any adverse effect on the low roughness values achieved by floating, for example by conducting the ceramization vertically or by an air-cushion ceramization, i.e. generally without any contact between the glass object being ceramized and a substrate.
Special advantages with regard to a very low surface roughness of the glass ceramic are achieved by a floated, ceramized aluminosilicate glass with the following composition in wt. % on an oxide basis: Li2O comprising three and two tenths to five weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; Na2O comprising zero to one and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; K2O comprising zero to one and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; xcexa3Na2O+K2O comprising two tenths to two weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; MgO comprising one tenth to two and two tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; CaO comprising zero to one and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; SrO comprising zero to one and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; BaO comprising zero to two and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; ZnO comprising zero to less than one and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; Al2O3 comprising nineteen to twenty-five weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; SiO2 comprising fifty-five to sixty-nine weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; TiO2 comprising one to five weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; ZrO2 comprising one to two and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; SnO2 comprising zero to less than one and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; xcexa3TiO2+ZrO2+SnO2 comprising two and five tenths to five weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; P2O5 comprising zero to three weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range.
In a second realization, the glass in one particularly preferred embodiment has a composition, in wt. % on an oxide basis, of: Li2O comprising three and five tenths to four and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; Na2O comprising two tenths to one weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; K2O comprising zero to eight tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; xcexa3Na2O+K2O comprising four tenths to one and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; MgO comprising three tenths to two weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; CaO comprising zero to one weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; SrO comprising zero to one weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; BaO comprising zero to two and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; ZnO comprising zero to one and weight percent within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; Al2O3 comprising nineteen to twenty-four weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; SiO2 comprising sixty to sixty-eight weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; TiO2 comprising one to two weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; ZrO2 comprising one and two tenths to two and two tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; SnO2 comprising zero to six tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; xcexa3TiO2+ZrO2+SnO2 comprising three to four and five tenths weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range; P2O5 comprising zero to two weight percent and within the range percentages in tenth of percent steps such that any tenth of a percent may be a limit of a diminished range.
This glass is used with particular advantage for the manufacture of the glass ceramic object claimed by the invention, because the corresponding surface is very easy to clean.
The glass ceramic object claimed by the invention can be used wherever ease of cleaning is necessary. It can be used for, among other things, oven door windows, in particular windows for doors for kitchen stoves, or for glass ceramic cooking surfaces of cooktops.
The above-discussed embodiments of the present invention will be described further herein below. When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is, the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, the Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintains that this application may include more than one patentably and non-obviously distinct invention. The Applicants hereby assert that the disclosure of this application may include more than one invention, and, in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.